Ethylene is a chemical precursor of significant commercial importance, for example as a raw material for the manufacture of polymers, ethylbenzene, styrene, and polystyrene, among other chemical products. The vast majority of ethylene commercially produced is derived from steam cracking of naphtha and/or ethane and/or propane. Ethylene may be obtained from the non-catalytic thermal cracking of saturated hydrocarbons, such as ethane and propane, and alternatively from thermal or steam cracking of heavier liquids such as naphtha and gas oil. Steam cracking produces a variety of other products, including diolefins and acetylene. The latter are costly to separate from the ethylene, usually by extractive distillation and/or selective hydrogenation to the corresponding mono-olefin, e.g. acetylene to ethylene. In addition, thermal cracking processes for olefin production are highly endothermic and sensitive to the quality of the feed stream. In traditional steam cracking, energy intensive separation steps would be used to remove some of the contaminants in the feed stream such as methane, hydrogen, CO, and C3 hydrocarbons. Some or all of these byproducts are frequently burned to partially offset the large energy deficit of the steam cracking process. The combustion of these byproducts as fuel produces significant CO2 and or NOx emissions. Furthermore, all of these processes require a large consumption of fuel and the construction and maintenance of large, capital-intensive and complex cracking furnaces to supply the heat.
Existing steam cracking processes generate ethylene by raising the feed (ethane or other hydrocarbons) to high enough temperature (700-1000° C.) in furnace tubes to thermally crack the hydrocarbons into olefins, especially ethylene and secondarily propylene, plus a range of other hydrocarbons, hydrogen and coke. The residence time must be very short, at a level measured in milliseconds, and the effluent must be quenched immediately, in order to maximize the desired olefins and minimize the undesired by-products. The pressure must be kept to a minimum, substantial steam dilution is required, and design features are critical for obtaining the best performance. As a result, the reaction conditions are very sensitive, and the furnaces are very expensive, with high fuel requirement due to both the high temperature and the high endothermicity of the cracking reactions. Frequent decoking is also a major requirement. Furthermore, furnace tubes must be replaced periodically.
Given the drawbacks of conventional steam cracking production of ethylene, significant research and effort has been invested in the development of alternative methods. One important alternative is to catalytically dehydrogenate ethane in the presence of oxygen to form ethylene. The process is called oxidative dehydrogenation (ODH). In this process, the product is largely limited to ethylene with small amounts of other byproducts such as methane, carbon monoxide, carbon dioxide, and other hydrocarbons. The effluent can also contain water (produced in the reaction plus whatever enters with the feed), residual ethane, some residual oxygen, and nitrogen if introduced with the oxygen (e.g., as air). The oxidative dehydrogenation (ODH) of ethane is thermodynamically favored and can be carried out at lower reaction temperatures that conventional steam cracking and without coke formation. Commercialization of ODH has been hindered low product selectivity at high ethane conversions. Further, the ODH byproducts, although not present in large enough amounts to warrant energy intensive separation, can be present in the feed and as byproducts of the ODH process. Separation can be energy intensive and, unlike steam cracking, ODH is energy efficient and does not require these byproducts (or a substitute fuel) to be burned to drive the process.
There remains a need for improved systems and methods for producing ethylene from ethane and ethane-containing fuels via oxidative dehydrogenation methods that do not produce or produce less of these byproducts.